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<tr class="t-nv-h2"><td colspan="5"> swap 、 forward 与 move</td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="forward.html" title="cpp/utility/forward"><span class="t-lines"><span>forward</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="move.html" title="cpp/utility/move"><span class="t-lines"><span>move</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="move_if_noexcept.html" title="cpp/utility/move if noexcept"><span class="t-lines"><span>move_if_noexcept</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 初等字符串转换</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="to_chars.html" title="cpp/utility/to chars"><span class="t-lines"><span>to_chars</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="from_chars.html" title="cpp/utility/from chars"><span class="t-lines"><span>from_chars</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="chars_format.html" title="cpp/utility/chars format"><span class="t-lines"><span>chars_format</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 类型运算</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="declval.html" title="cpp/utility/declval"><span class="t-lines"><span>declval</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="as_const.html" title="cpp/utility/as const"><span class="t-lines"><span>as_const</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="launder.html" title="cpp/utility/launder"><span class="t-lines"><span>launder</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><strong class="selflink"><tt>std::hash</tt></strong><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="hash/hash.html" title="cpp/utility/hash/hash"><span class="t-lines"><span>hash::hash</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="hash/operator%28%29.html" title="cpp/utility/hash/operator()"><span class="t-lines"><span>hash::operator()</span></span></a></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../header/functional.html" title="cpp/header/functional">&lt;functional&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> Key <span class="sy1">&gt;</span><br>
<span class="kw1">struct</span> hash<span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>此模板的每个特化为<i>启用</i>（“无污染”）或为<i>禁用</i>（“中毒”）。对于每个既非库亦非用户提供的数据类型的<code>Key</code>启用特化的函数 <code>std::hash&lt;Key&gt;</code>, 特化存在且被禁用。被禁用特化不满足<a href="../named_req/Hash.html" title="cpp/named req/Hash"><i>散列</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Hash)</span> </a>，不满足<a href="../named_req/FunctionObject.html" title="cpp/named req/FunctionObject"><i>函数对象</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(FunctionObject)</span> </a>，且 <span class="t-lc"><a href="../types/is_default_constructible.html" title="cpp/types/is default constructible">std::is_default_constructible_v</a></span> 、 <span class="t-lc"><a href="../types/is_copy_constructible.html" title="cpp/types/is copy constructible">std::is_copy_constructible_v</a></span> 、 <span class="t-lc"><a href="../types/is_move_constructible.html" title="cpp/types/is move constructible">std::is_move_constructible_v</a></span> 、 <span class="t-lc"><a href="../types/is_copy_assignable.html" title="cpp/types/is copy assignable">std::is_copy_assignable_v</a></span> 、 <span class="t-lc"><a href="../types/is_move_assignable.html" title="cpp/types/is move assignable">std::is_move_assignable_v</a></span> 全为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。换言之，它们存在但无法使用。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<p>hash 函数模板<span class="t-rev-inl t-since-cxx17"><span>的<i>启用</i>的特化</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>定义一个实现<a href="https://en.wikipedia.org/wiki/Hash_function" class="extiw" title="enwiki:Hash function">散列函数</a>的函数对象。此函数对象的实例满足<a href="../named_req/Hash.html" title="cpp/named req/Hash"><i>散列</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Hash)</span> </a>。特别是它们定义满足下列条件的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">(</span><span class="br0">)</span></span></span> ：
</p><p>1. 接收 <code>Key</code> 类型的单个参数
</p><p>2. 返回表示参数哈希值的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a></span></span> 类型。
</p><p>3. 调用时不抛出异常。
</p><p>4. 对于二个相等的参数 <code>k1</code> 与 <code>k2</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span>Key<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k1<span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span>Key<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k2<span class="br0">)</span></span></span> 。
</p><p>5. 对于二个相异而不相等的参数 <code>k1</code> 与 <code>k2</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span>Key<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k1<span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span>Key<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k2<span class="br0">)</span></span></span> 的概率应非常小，接近 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu16">1.0</span><span class="sy2">/</span><a href="../types/numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1">&lt;</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">max</span><span class="br0">(</span><span class="br0">)</span></span></span> 。
</p><p>库函数提供的 <code>hash</code> 的所有显式和部分特化<a href="../named_req/DefaultConstructible.html" title="cpp/named req/DefaultConstructible"><i>可默认构造</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> </a>、<a href="../named_req/CopyAssignable.html" title="cpp/named req/CopyAssignable"><i>可复制赋值</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span> </a>、<a href="../named_req/Swappable.html" title="cpp/named req/Swappable"><i>可交换</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Swappable)</span> </a>且<a href="../named_req/Destructible.html" title="cpp/named req/Destructible"><i>可析构</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span> </a>。用户提供的 <code>hash</code> 特化亦必须满足这些要求。
</p><p>无序关联容器 <span class="t-lc"><a href="../container/unordered_set.html" title="cpp/container/unordered set">std::unordered_set</a></span> 、 <span class="t-lc"><a href="../container/unordered_multiset.html" title="cpp/container/unordered multiset">std::unordered_multiset</a></span> 、 <span class="t-lc"><a href="../container/unordered_map.html" title="cpp/container/unordered map">std::unordered_map</a></span> 、 <span class="t-lc"><a href="../container/unordered_multimap.html" title="cpp/container/unordered multimap">std::unordered_multimap</a></span> 以该模板 <span class="t-lc"><strong class="selflink">std::hash</strong></span> 的特化为默认哈希函数。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>除了上述指定的情况外，哈希函数的实现方式是实现定义的。（比如， gcc 的<a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B11/hashtable_c%2B%2B0x.cc">实现</a>）。需要注意，某些哈希函数的实现过于简单，甚至是直接将对象映射至自身。这可能会造成严重的后果。换言之，这些哈希函数可以用于无序关联容器，但它们不一定安全/抗碰撞。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>哈希函数仅要求在程序的单次执行中对同样的输入返回同样的结果；这允许避免碰撞拒绝服务攻击的加盐哈希。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td></tr>
</table>
<p>没有对 C 字符串的特化。 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span><span class="sy1">&gt;</span></span></span> 产生指针值（内存地址）的哈希，它不检验任何字符数组的内容。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx20"><td>
<h3> <span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">  
</td></tr>
<tr class="t-dsc-hitem">
<td> 成员类型
</td>
<td> 定义
</td></tr>


<tr class="t-dsc">
<td>  <code>argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span>
</td>
<td>  <code>Key</code>
</td></tr>

<tr class="t-dsc">
<td>  <code>result_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span>
</td>
<td>  <span class="t-lc"><a href="../types/size_t.html" title="cpp/types/size t">std::size_t</a></span>
</td></tr>
</table>
</td>
<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div t-dsc-member-nobold-div"><div><a href="hash/hash.html" title="cpp/utility/hash/hash"> <span class="t-lines"><span>(构造函数)</span></span></a></div></div>
</td>
<td>  构造哈希函数对象 <br> <span class="t-mark">(公开成员函数)</span>
</td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="hash/operator%28%29.html" title="cpp/utility/hash/operator()"> <span class="t-lines"><span>operator()</span></span></a></div></div>
</td>
<td>  计算参数的哈希 <br> <span class="t-mark">(公开成员函数)</span>
</td></tr>
</table>
<h3><span class="mw-headline" id=".E5.9F.BA.E6.9C.AC.E7.B1.BB.E5.9E.8B.E7.9A.84.E6.A0.87.E5.87.86.E7.89.B9.E5.8C.96">基本类型的标准特化</span></h3>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../header/functional.html" title="cpp/header/functional">&lt;functional&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">bool</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<p><span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">signed</span> <span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span>char8_t<span class="sy1">&gt;</span><span class="sy4">;</span>        <span class="co1">// C++20</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">char16_t</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">char32_t</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">wchar_t</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">short</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">unsigned</span> <span class="kw4">short</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">unsigned</span> <span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">long</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">long</span> <span class="kw4">long</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">unsigned</span> <span class="kw4">long</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">unsigned</span> <span class="kw4">long</span> <span class="kw4">long</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">float</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><span class="kw4">long</span> <span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">;</span><br>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span><a href="../types/nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// C++17</span><br>
</p>
<span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T <span class="sy1">&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span>T<span class="sy2">*</span><span class="sy1">&gt;</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>在上述之外，标准库对所有（有作用域或无作用域）枚举类型提供特化。可以（但不要求）实现为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span><a href="../types/underlying_type.html"><span class="kw637">std::<span class="me2">underlying_type</span></span></a><span class="sy1">&lt;</span>Enum<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span><span class="sy1">&gt;</span></span></span>
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td></tr>
<tr class="t-rev t-since-cxx17"><td>
<p>标准库提供 <code>std::hash</code> 对 <span class="t-lc"><a href="../types/nullptr_t.html" title="cpp/types/nullptr t">std::nullptr_t</a></span> 和所有无 cv 限定算术类型（包含任何扩展整数类型）、所有枚举类型和所有指针类型的启用特化。
</p><p>每个声明模板 <code>std::hash</code> 标准库头文件提供上述启用特化。这些头文件包括 <a href="../header/string.html" title="cpp/header/string"><tt>&lt;string&gt;</tt></a> 、 <a href="../header/system_error.html" title="cpp/header/system error"><tt>&lt;system_error&gt;</tt></a> 、 <a href="../header/bitset.html" title="cpp/header/bitset"><tt>&lt;bitset&gt;</tt></a> 、 <a href="../header/memory.html" title="cpp/header/memory"><tt>&lt;memory&gt;</tt></a> 、 <a href="../header/typeindex.html" title="cpp/header/typeindex"><tt>&lt;typeindex&gt;</tt></a> 、 <a href="../header/vector.html" title="cpp/header/vector"><tt>&lt;vector&gt;</tt></a> 、 <a href="../header/thread.html" title="cpp/header/thread"><tt>&lt;thread&gt;</tt></a> 、 <a href="../header/optional.html" title="cpp/header/optional"><tt>&lt;optional&gt;</tt></a> 、 <a href="../header/variant.html" title="cpp/header/variant"><tt>&lt;variant&gt;</tt></a> 、 <a href="../header/string_view.html" title="cpp/header/string view"><tt>&lt;string_view&gt;</tt></a><span class="t-rev-inl t-since-cxx20"><span> 、 <a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/header/coroutine&amp;action=edit&amp;redlink=1" class="new" title="cpp/header/coroutine（页面不存在）"><tt>&lt;coroutine&gt;</tt></a></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 。
</p><p>此模板的标准库特化的所有函数均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span></span></span> ，除了 <a href="optional/hash.html" title="cpp/utility/optional/hash"><tt>std::hash&lt;std::optional&gt;</tt></a> 、 <a href="variant/hash.html" title="cpp/utility/variant/hash"><tt>std::hash&lt;std::variant&gt;</tt></a> 和 <a href="../memory/unique_ptr/hash.html" title="cpp/memory/unique ptr/hash"><tt>std::hash&lt;std::unique_ptr&gt;</tt></a> 的成员函数。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.BA.93.E7.B1.BB.E5.9E.8B.E7.9A.84.E6.A0.87.E5.87.86.E7.89.B9.E5.8C.96">库类型的标准特化</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../string/basic_string/hash.html" title="cpp/string/basic string/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::u8string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::u16string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::u32string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::wstring&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::pmr::string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::pmr::u8string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::pmr::u16string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::pmr::u32string&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::pmr::wstring&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div>
</td>
<td>   string 的哈希支持  <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../error/error_code/hash.html" title="cpp/error/error code/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::error_code&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>  <span class="t-lc"><a href="../error/error_code.html" title="cpp/error/error code">std::error_code</a></span> 的散列支持 <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="bitset/hash.html" title="cpp/utility/bitset/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::bitset&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>  <span class="t-lc"><a href="bitset.html" title="cpp/utility/bitset">std::bitset</a></span> 的散列支持 <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../memory/unique_ptr/hash.html" title="cpp/memory/unique ptr/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::unique_ptr&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>  <span class="t-lc"><a href="../memory/unique_ptr.html" title="cpp/memory/unique ptr">std::unique_ptr</a></span> 的散列支持  <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../memory/shared_ptr/hash.html" title="cpp/memory/shared ptr/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::shared_ptr&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>  <span class="t-lc"><a href="../memory/shared_ptr.html" title="cpp/memory/shared ptr">std::shared_ptr</a></span> 的散列支持 <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../types/type_index/hash.html" title="cpp/types/type index/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::type_index&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>   <span class="t-lc"><a href="../types/type_index.html" title="cpp/types/type index">std::type_index</a></span> 的散列支持 <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../container/vector_bool/hash.html" title="cpp/container/vector bool/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::vector&lt;bool&gt;&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>   <a href="../container/vector_bool.html" title="cpp/container/vector bool"><tt>std::vector&lt;bool&gt;</tt></a> 的哈希支持 <br> <span class="t-mark">(类模板特化)</span>
</td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../thread/thread/id/hash.html" title="cpp/thread/thread/id/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::thread::id&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>   <span class="t-lc"><a href="../thread/thread/id.html" title="cpp/thread/thread/id">std::thread::id</a></span> 的哈希支持 <br> <span class="t-mark">(类模板特化)</span>
</td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="optional/hash.html" title="cpp/utility/optional/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::optional&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div>
</td>
<td>   特化 <span class="t-lc"><strong class="selflink">std::hash</strong></span> 算法  <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="variant/hash.html" title="cpp/utility/variant/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::variant&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div>
</td>
<td>  特化 <span class="t-lc"><strong class="selflink">std::hash</strong></span> 算法  <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../string/basic_string_view/hash.html" title="cpp/string/basic string view/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::string_view&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::wstring_view&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::u8string_view&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::u16string_view&gt;</span></span><span>std::hash<span class="t-dsc-small">&lt;std::u32string_view&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div>
</td>
<td>   string_view 的散列支持  <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../error/error_condition/hash.html" title="cpp/error/error condition/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::error_condition&gt;</span></span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div>
</td>
<td>   <span class="t-lc"><a href="../error/error_condition.html" title="cpp/error/error condition">std::error_condition</a></span> 的哈希支持  <br> <span class="t-mark">(类模板特化)</span> </td></tr>
</table>
<p>注意：对 <code>std::pair</code> 和标准容器类型的特化，还有组合哈希的工具函数可用于 <a rel="nofollow" class="external text" href="http://www.boost.org/doc/libs/release/doc/html/hash/reference.html">boost.hash</a>
</p><p><br>
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;iomanip&gt;</span>
<span class="co2">#include &lt;functional&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;unordered_set&gt;</span>
 
<span class="kw1">struct</span> S <span class="br0">{</span>
    <a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> first_name<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> last_name<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&amp;</span> lhs, <span class="kw4">const</span> S<span class="sy3">&amp;</span> rhs<span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">return</span> lhs.<span class="me1">first_name</span> <span class="sy1">==</span> rhs.<span class="me1">first_name</span> <span class="sy3">&amp;&amp;</span> lhs.<span class="me1">last_name</span> <span class="sy1">==</span> rhs.<span class="me1">last_name</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// 自定义哈希能是独立函数对象：</span>
<span class="kw1">struct</span> MyHash
<span class="br0">{</span>
    <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>S <span class="kw4">const</span><span class="sy3">&amp;</span> s<span class="br0">)</span> <span class="kw4">const</span> 
    <span class="br0">{</span>
        <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> h1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span><a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>s.<span class="me1">first_name</span><span class="br0">)</span><span class="sy4">;</span>
        <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> h2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span><a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>s.<span class="me1">last_name</span><span class="br0">)</span><span class="sy4">;</span>
        <span class="kw1">return</span> h1 <span class="sy3">^</span> <span class="br0">(</span>h2 <span class="sy1">&lt;&lt;</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 或使用 boost::hash_combine （见讨论）</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// std::hash 的自定义特化能注入 namespace std</span>
<span class="kw1">namespace</span> std
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">struct</span> hash<span class="sy1">&lt;</span>S<span class="sy1">&gt;</span>
    <span class="br0">{</span>
        <span class="kw1">typedef</span> S argument_type<span class="sy4">;</span>
        <span class="kw1">typedef</span> <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> result_type<span class="sy4">;</span>
        result_type operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>argument_type <span class="kw4">const</span><span class="sy3">&amp;</span> s<span class="br0">)</span> <span class="kw4">const</span>
        <span class="br0">{</span>
            result_type <span class="kw4">const</span> h1 <span class="br0">(</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span><a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>s.<span class="me1">first_name</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
            result_type <span class="kw4">const</span> h2 <span class="br0">(</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span><a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>s.<span class="me1">last_name</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
            <span class="kw1">return</span> h1 <span class="sy3">^</span> <span class="br0">(</span>h2 <span class="sy1">&lt;&lt;</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 或使用 boost::hash_combine （见讨论）</span>
        <span class="br0">}</span>
    <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
 
    <a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Meet the new boss..."</span><span class="sy4">;</span>
    <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> str_hash <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span><a href="../string/basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"hash("</span> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>str<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">") = "</span> <span class="sy1">&lt;&lt;</span> str_hash <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    S obj <span class="sy1">=</span> <span class="br0">{</span> <span class="st0">"Hubert"</span>, <span class="st0">"Farnsworth"</span><span class="br0">}</span><span class="sy4">;</span>
    <span class="co1">// 使用独立的函数对象</span>
    <a href="../io/cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"hash("</span> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>obj.<span class="me1">first_name</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">','</span> 
               <span class="sy1">&lt;&lt;</span> <a href="../io/manip/quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>obj.<span class="me1">last_name</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">") = "</span>
               <span class="sy1">&lt;&lt;</span> MyHash<span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>obj<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">" (using MyHash)<span class="es1">\n</span>                           or "</span>
               <span class="sy1">&lt;&lt;</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1">&lt;</span>S<span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>obj<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">" (using std::hash) "</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="co1">// 自定义哈希令在无序容器中使用自定义类型可行</span>
    <span class="co1">// 此示例将使用注入的 std::hash 特化，</span>
    <span class="co1">// 若要使用 MyHash 替代，则将其作为第二模板参数传递</span>
    <a href="../container/unordered_set.html"><span class="kw1288">std::<span class="me2">unordered_set</span></span></a><span class="sy1">&lt;</span>S<span class="sy1">&gt;</span> names <span class="sy1">=</span> <span class="br0">{</span>obj, <span class="br0">{</span><span class="st0">"Bender"</span>, <span class="st0">"Rodriguez"</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">"Leela"</span>, <span class="st0">"Turanga"</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
    <span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;</span> s<span class="sy4">:</span> names<span class="br0">)</span>
        <a href="../io/cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>s.<span class="me1">first_name</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>s.<span class="me1">last_name</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">hash("Meet the new boss...") = 1861821886482076440
hash("Hubert","Farnsworth") = 17622465712001802105 (using MyHash)
                           or 17622465712001802105 (using std::hash) 
"Leela" "Turanga"
"Bender" "Rodriguez"
"Hubert" "Farnsworth"</pre></div></div> 
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